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Understanding The Gear Effect

Drivers over the past couple years have all been touting terms like “optimized flight” “low center of gravity” and “more stable on off-center hits”.  In an engineers quest to achieve perfection in a driver, we’ve seen quite a variety of designs, including square heads (Callaway FTi), triangular (Titleist 907 D1), and now even cavity back (Nike Covert).  When it comes down to it though, engineers are simply manipulating the clubs center of gravity to help optimize its gear effect. The gear effect is one of the crucial elements of all driver designs made today, and very few golfers understand how it affects their golf ball’s direction.  In this post we’re going to dive deeper into how the gear effect works and how it’s affecting your game.

Have you ever wondered why your driver clubface is curved and your irons are not? The answer has to do with the gear effect.  Engineers have long been aware that toe hits on a driver tend to impart hook spin, while heel hits impact slice spin.  This is because off-center hits causes drivers to pivot around the clubs center of gravity producing spin on the ball.  Engineers started to combat this effect by developing rounded clubfaces – called a “bulge”.  The bulge on modern day drivers sends toe hits off to the right at impact, and heel shots to the left to minimize the effects of the gear effect and help keep a ball on target.  The amount of bulge a driver has is proportional to the position of its center of gravity.

Bulge Effect Driver Golf

Irons on the other hand do not have a bulge and the reason why comes down to placement of their center of gravity. An iron’s center of gravity is very close to the impact point of the ball, even for cavity-back irons.  The closer a club’s center of gravity is to the point of impact, the less the gear effect comes into play. If an iron were to be given a horizontal bulge across the face, off-center hits would start off-line, much like a driver, but then there wouldn’t be enough corrective sidespin caused by the gear effect to get the ball back on line.

Bulge Effect Irons

Where this all becomes very interesting is when you start factoring in swing speeds. As an instructor of the game we were originally taught the nine ball flight laws, which ultimately describes the nine possible ball flights you can get through a combination of swing path and face angles. These laws state that an open clubface will always produce a slice and a closed clubface produces a hook. While this is true for center hits, off-center hits turn these rules inside out, and this becomes really evident the faster you swing.

Golf club engineers have known this for some time.  When the first adjustable driver came out from TaylorMade, they noticed swings at tour speeds (115MPH) could produce 30+ of curve, while swing speeds around 85 MPH would only produce 6 yards of curve.  The gear effect operates similarly, the faster the swing speed, the more pronounced the response will be. Dustin Johnson’s swing was recently analyzed in slow motion after he snap hooked a tee-shot at the Hyundai Tournament of Champions.  The announcers were stunned to see his clubface was wide open at impact and not closed. This is an example of how the gear effect at high swing speeds can produce rather unexpected results.

Dustin Johnson swings his driver at 124 MPH on average, and on this particular swing he hit the ball deep on the toe, causing the club to torque strongly around its rearward center of gravity, imparting a ton of hook spin on the ball.  The ball started right and then severely hooked left.

While all this may be interesting, you likely asking yourself – “yeah, but how does this help my game”. Well the answer is, understanding your swing faults.  Its long been taught that slices are caused by open clubfaces at impact, and the same is true for closed clubfaces and hooks. But, what is never brought up is the role off-center hits have in ball flight.  Understanding how off-center hits react on impact can better help you discover what’s going wrong in your swing.  Better still, understanding that the same swing fault can cause a toed driver to send the ball in the completely opposite direction as a toed iron shot.

I challenge you to head out to the range and purposely hit balls off the heel and toe of your drivers and irons and take note of your ball direction, flight, and the feel of the shot.  You’ll likely be surprised to see the role the gear effect plays on your clubs.


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